1. Field of the Invention
The present invention relates to a synchronous signal generating method, recording apparatus, transmitting apparatus, recording medium, and transmission medium in which a p-bit input data word is converted to a q-bit (q>p) code word using a plurality of coding tables, and a code word string produced by directly coupling the code words is recorded in a recording medium such as an optical disk and magnetic disk and reproduced, or transmitted via a transmitting portion, in which a coding rate is raised according to high density of recording, a synchronous signal for decoding reproduction data is inserted into every predetermined number of code words in a string of the code words, and the synchronous signal is surely separated from the code word string so as to reproduce digital data with high precision.
2. Description of the Related Art
Generally, a pit length to be recorded in an optical disk has a restriction on a minimum run length (minimum pit or land length) due to optical transmission characteristic for recording and reproducing and physical restraint about pit generation and further, a restriction on a maximum run length (maximum pit or land length) for a reason of facilitation of clock generation. Further for protection of a servo band, and the like, it is necessary to modulate signals to be recorded so that the signals have a suppression characteristic of low-pass components of the signals.
As a conventional modulation method satisfying these restrictions, in which the minimum run length (hereinafter referred to also as a minimum reverse interval) is 3T (T=a period of channel bit), and a maximum run length (hereinafter referred to also as a maximum reverse interval) is 11T, an 8 to 14 modulation (EFM) method for use in a compact disk (CD), and EFM+ method for use in a digital versatile disk (DVD) have been well known.
First, in EFM modulation for use in the compact disk (CD), inputted 8-bit (1 byte) digital data is converted to 14-bit run length limited code (hereinafter referred to as a code word) satisfying the run length restriction such that the minimum run length is 3T and the maximum run length is 11T. Furthermore, a 3-bit connection bit for controlling a digital sum value (DSV) and holding a run length restriction rule is attached between the converted code words so that an EFM modulated signal is generated.
In this case, for the minimum run length of 3T, a minimum number of “0” included between logical values “1” and “1” in code words is d=2. On the other hand, for the maximum run length of 11T, a maximum number of “0” included between the logical values “1” and “1” in the code words is k=10. Moreover, the 3-bit connection bit disposed for controlling the DSV and holding the run length restriction rule in order to reduce a DC component and low-pass component of the modulated signal is connected between the 14-bit code words. Thereby, the EFM modulated signal satisfies a run length restriction rule RLL (d, k)=RLL (2, 10) that the minimum run length is 3T and the maximum run length is 11T.
In a CD, the EFM modulated signal with a synchronous signal added is recorded. In a code word string of the EFM modulated signal, a repeating pattern of the maximum run length 11T, that is, a repeating pattern of 11T-11T is avoided so as to set the repeating pattern of 11T-11T as a synchronous signal.
A CD player for reproducing the CD detects the repeating pattern of 11T-11T from signals read from the CD, thereby sampling the synchronous signal.
Next, in the EFM+ method used for the digital versatile disk (DVD), the inputted 8-bit digital data is converted to a 16-bit code word, these code words are directly coupled to one another without using any connection bit, and thereby 8 to 16 modulation is executed so as to satisfy the run length restriction rule RLL (2, 10) that the minimum run length is 3T and the maximum run length is 11T.
The CD player can, as described above, sample the synchronous signal of the repeating pattern of 11T-11T from signals read from the CD. However, reading information on a recording medium such as an optical disk of much higher density than a DVD or a transmission medium for transmitting modulated signals at higher density would be largely affected by interference between codes. If the synchronous signal used for the CD is used in the medium, the synchronous signal of the repeating pattern of 11T-11T may change to a pattern of 11T-10T or 10T-11T to be read. Conversely, it may occur that a data pattern of 10T-11T or 11T-10T of the EFM modulated signal changes to the repeating pattern of 11T-11T and is erroneously detected as the synchronous signal.
This increases error generation frequency in detecting the synchronous signal in high density recording on a recording medium such as an optical disk or in high density data transmission, leading to easy generation of a burst error due to the loss of synchronization.
Although the DVD using the EFM+ method solves the above problem because the synchronous signal is set at 14T-4T, it causes a lower coding rate.